The use of structured substrates has become increasingly important in a variety of applications. Their use has become especially important in applications where very small structures are desired and where dimensional tolerances are very tight (for example, in liquid crystal display substrates or in high definition large screen television displays). During the fabrication of devices employing structured substrates, it is often necessary to precisely modify the surface of the substrate in certain patterns commensurate with the structures on the surface. Such modification can include removing material from selected areas, depositing material on selected areas, or otherwise physically or chemically modifying selected areas.
Conventional lithography techniques may be successfully employed to modify selected areas of surfaces. Such techniques involve first coating a surface with a photoresist material. The photoresist is then selectively exposed to light through a mask so that only those areas of the photoresist not covered by the mask are illuminated. The photoresist in either the illuminated or unilluminated areas is subsequently removed by known techniques (depending on whether a positive or negative photoresist is used), thereby exposing only those portions of the underlying surface to be modified. After modification of the exposed surface (i.e., via etching, deposition, etc.), the remaining photoresist is removed to yield a patterned surface.
Difficulties arise when the surface to be modified is a structured surface. A structured surface is one that comprises a plurality of well-defined protrusions, indentations, or both on an otherwise substantially uniform surface. Often the structures are formed in a repeating pattern as those formed via known microreplication techniques. When modifying only the protrusions, for example, on a structured substrate, conventional lithography techniques require that the mask exactly match the pattern of the protrusions and that the mask is perfectly aligned with the protrusions. This is especially difficult for large area substrates. When using even high precision masks, errors will occur despite a high degree of care during alignment, and these errors will be compounded on larger area substrates. For example, a high precision mask designed to match a 100 .mu.m repeating pattern to within 0.1 .mu.m on a 100 cm.sup.2 structured substrate would unavoidably result in misalignment of the mask on at least some areas of the substrate. Creating very high precision masks and aligning those masks with equal or better precision can thus be a very difficult, time-consuming, and costly process, and for large enough substrates will simple be impossible. These concerns are multiplied by the fact that a new mask must be designed to match each new substrate or each new set of structures to be modified.
Although lithography techniques are well-known and the art of employing these techniques is both varied and mature, such techniques are incapable of meeting the unique challenges presented when developing patterning processes for structured surfaces at a reasonable cost, especially as surface structures are pushed to smaller and smaller dimensions while substrate areas are pushed to larger and larger dimensions. In particular, conventional lithography techniques involve step-and-repeat process steps and are not adaptable to roll-to-roll continuous processes to pattern structured substrates.
In U.S. Pat. No. 5,382,317 (Thomas), a method of selectively applying a coating to either the upper or lower surface of a bilevel substrate was disclosed. Using this technique, one could modify the protrusions on a structured substrate by first applying a layer of photoresist to the surface and then removing residual photoresist from the tops of the protrusions by using a blade. The protrusions can then be modified while the rest of the surface is protected by photoresist. This technique is limited, however, to surfaces having protrusions that rise to substantially the same level and where the tops of the protrusions are substantially flat. If either one of these conditions does not hold, it is likely that the residual coating present on the protrusion tops will not be completely removed by the blade, and the desired modifications will be incomplete. Even when these conditions do hold, there is no way to ensure that the blade will not leave at least some resist behind. Moreover, large area surfaces pose additional problems because the likelihood for height variations in the protrusions increases with area. Also, this technique is not well suited for surfaces having protrusions that rise only a small distance (i.e. about or less than 2 .mu.m high) because there is a danger that when removing the coating from the protrusion tops that a portion of the coating between the protrusions may be removed also.
Another method for selectively exposing the tops of protrusions on a structured substrate is disclosed in EP 0 564 364 A2 (McFadden). In this method, a structured substrate is coated with a photoresist and a prism is placed on the substrate so that the prism rests on the protrusion tops, being in contact with only the photoresist that is present on the protrusions. The prism is then irradiated with a collimated light beam as from a laser. At a certain incident angle, the light in the prism will satisfy the conditions for total internal reflection at the surface of the prism adjacent to the substrate. Between protrusions, the prism interface is with air, a lower index medium, and so the light will be totally internally reflected. At the protrusions, the prism interface is with the photoresist which can be chosen to have the same or higher index as the prism so that at least a portion of the light will be transmitted. In this way, the photoresist on top of the protrusions is irradiated while the photoresist between protrusions is left unmodified. The photoresist can then be selectively removed either from the protrusion tops or from the valleys between the protrusions. One difficulty with this method is that the protrusions must be substantially the same height and the photoresist coating on the protrusions must be substantially flat. If there is a deviation in the heights of protrusions, the prism may not contact all the protrusion tops so that not all protrusions will be irradiated. In addition, for substrates larger than a few inches wide, multiple exposures and/or multiple prisms may be required because the entire surface cannot be covered using one prism. Finally, light rays that irradiate the photoresist on the protrusion tops can propagate in the photoresist layer or in the substrate and be trapped by total internal reflection. These trapped light rays may be propagated to areas between protrusions, thus irradiating photoresist in undesired areas.